This invention relates to a gamma correction circuit having its frequency characteristic extended to a broader band.
A gamma correction circuit comprising the combination of a linear amplifier and a non-linear amplifier and capable of continuously varying the gamma value is described in, for example, a book entitled "Gazo Denshi Kairo (Video Electronic Circuits)" published by Corona-sha Inc. in Japan on Jan. 20, 1979. Such a circuit will be explained with reference to FIG. 1.
Referring to FIG. 1, a current source 5 supplies a video signal current to a linear amplifier 6 and a non-linear amplifier 7, which include non-linear elements or diodes 8a, 8b, linear elements or resistors 9a, 9b and common collector transistors 10a, 10b, respectively. A variable resistor 11 for setting the gamma value is connected between the emitters of the transistors 10a and 10b.
The video signal current of current value 2i.sub.S supplied from the current source 5 is equally divided into halves by the two lines composed of the diodes 8a, 8b and resistors 9a, 9b respectively, so that the current of current value i.sub.S flows through each of the two lines. This current of current value i.sub.S flows through the series-connected linear element and non-linear element in each line, resulting in a voltage drop across these elements. Output potentials e.sub.L and e.sub.N determined by the voltage drop across those elements appear at the emitters of the transistors 10a and 10b respectively, and these potentials are applied to the fixed terminals respectively of the variable resistor 11.
The internal resistance value of the diode 8b, which is the non-linear element, is dependent upon the current value i.sub.S. Therefore, the output e.sub.N of the non-linear amplifier 7 varies in non-linear relation to the input voltage (when the current of current value 2i.sub.S is converted into the voltage by the linear element). Thus, the gamma value can be corrected as desired by suitably varying the mixing ratio of the output e.sub.N of the non-linear amplifier 7 and the output e.sub.L of the linear amplifier 6.
For the purpose of ensuring completely satisfactory operation of the circuit, it is required that the output e.sub.N of the non-linear amplifier 7 is equal to the output e.sub.L of the linear amplifier 6 at the white peak level and black level of a video signal. Thus, when the current value 2i.sub.S of the video signal current is selected to be 2i.sub.S =0 at the black level, the value of each of the outputs e.sub.N and e.sub.L is equal to the power supply voltage V.sub.cc, and the outputs e.sub.N and e.sub.L can be made equal to each other. On the other hand, at the white peak level, it is required that the resistance value R.sub.N of the diodes 8a and 8b is equal to the resistance value R.sub.L of the resistors 9a and 9b at the existing current value i.sub.S.
Generally, the junction capacitance of a transistor increases sharply with the decrease in the level of a voltage applied thereacross. Therefore, in the circuit shown in FIG. 1, the base-collector junction capacitance of the transistors 10a and 10b increases sharply as the signal level approaches the black level. The increase in the base-collector junction capacitance of the transistors 10a and 10b means the increase in the input capacitance of the linear amplifier 6 and non-linear amplifier 7. Consequently, at the black level, the time constant determined by this input capacitance and the resistance value of the input circuit including the elements having the resistance values R.sub.N and R.sub.L increases to lower the cutoff frequency fc of the gamma correction circuit. Therefore, the prior art gamma correction circuit has been defective in that it is not applicable to a display unit requiring a high resolution.